What is biological magnification and how does it relate to the health warnings given to pregnant women about the consumption of certain fish?

Biological magnification refers to a phenomenon where some of the substances like heavy metals or pesticides move up the food chain, make their way into the lakes or rivers, and are eventually consumed by aquatic species like fish. This, in turn, is further consumed by large species like large birds, humans or animals.  

This is associated with the health warnings given to pregnant women as too much eating of some kind of fish may influence the immune systems, that is, may affect the health of women who are pregnant.  

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.